Incorrect value in calculating SPI by MDM.

Hello, I have a question. I am working with MDM software for calculate SPI, but I got a problem when I try calculate monthly drought, sometimes I got the values is 1.00e+300. how can i solve this proplem?. thank you so much.

MDM, DMAP and RDIT tool calculate SPI by using Gamma distribution. Sometimes during the calculation, the values change to NaN (1.00e+300) and we have a value in this month(or year).

First of all, Change the rainfall less than 1 with 0 and calculate the index again. The Gamma distribution is NaN for value between [0 to 1)

Please check the month for dry or wet and if the month was dry change the value to -4 and if the month was wet change it to +4. If the month has a big value of rainfall so the month is wet and if the month has a low value of rainfall so it is a dry month.